This invention relates generally to a piezoelectric device, and more specifically to controlling the displacement of the piezoelectric device over a temperature range.
Piezoelectric devices, such as piezoelectric actuators, generally consist of a piezoelectric material that deforms when an electric field, e.g., a driving field, is applied across it. Additional materials may be bonded with the piezoelectric material, such as metallic layers that act as electrodes, insulating materials to prevent current from flowing between particular areas of the device, and adhesives to bond the various layers together.
In simplified terms, piezoelectric materials are comprised of many dipole unit cells. FIG. 1 symbolically depicts a unit cell 10 of a piezoelectric material. When an electric field E1 is applied in the direction shown, the unit cell grows in the y axis and shrinks in the x axis, in essence becoming tall and thin. Conversely, when an electric field E2 is applied in the direction shown, the unit cell shrinks along the y axis and grows along the x axis, in essence, becoming short and fat.
As the unit cell 10 becomes colder, the piezoelectric effect, i.e., response to the application of an electric field, decreases. Thus, for a given magnitude of an electric field, the unit cell will not grow/shrink as much as it did when it was warmer. As a practical matter, for the same electric field applied, this decreases the stroke of the piezoelectric device. For example, for some piezoelectric materials, a 35% loss of stroke was found when the temperature changed from 25 degrees Celsius to xe2x88x9240 degrees Celsius.
To complicate matters, piezoelectric materials have predetermined operating ranges for the electric fields that are applied to them. Applying an electric field in excess of the maximum operating range may cause excessive domain wall motion, i.e., dipole switching for a group of commonly polarized/aligned unit cells, causing fatigue and micro-cracking. This may significantly decrease the performance and life of the piezoelectric material. Even higher electric fields can cause dielectric strength to become a concern.
The present invention provides methods and apparatus for controlling an electric field across a piezoelectric device. A temperature determining device transmits a temperature signal indicative of a temperature of the piezoelectric device. A control device receives the temperature signal and a first control signal indicative of a desired displacement of the piezoelectric device. The control device transmits a second control signal to the piezoelectric device as a function of the temperature signal and the control signal. The second control signal creates the electric field across the piezoelectric device, when the temperature of the piezoelectric device is below a predetermined threshold, having a magnitude above a normal operating range for the piezoelectric device.